As shown in FIG. 1, a conventional fuel assembly 10 of a nuclear reactor, such as a Boiling Water Reactor (BWR), may include an outer channel 12 surrounding an upper tie plate 14 and a lower tie plate 16. A plurality of full length fuel rods 18 and/or part length fuel rods 19 may be arranged in a matrix within the fuel assembly 10 and pass through a plurality of spacers (also known as spacer grids) 15 axially spaced one from the other and maintaining the rods 18, 19 in the given matrix thereof. The fuel rods 18 and 19 are generally continuous from their base to terminal, which, in the case of the full length fuel rod 18, is from the lower tie plate 16 to the upper tie plate 14. Outer channel 12 encloses the fuel rods 18/19 within the assembly 10 and maintains water or other coolant flow within assembly 10 about fuel rods 18/19 and in contact with the fuel rods 18/19 to facilitate heat transfer from the fuel to the coolant. Outer channel 12 is traditionally uniform in mechanical design and material for each other assembly 10 provided to a particular core, to aid in assembly design standardization and manufacturing simplicity. Outer channel 12 may be fabricated conventionally of a material compatible with the operating nuclear reactor environment, such as a Zircaloy-2.
As shown in FIG. 2, a conventional reactor core, such as a BWR core, may include a plurality of cells 40 in the reactor core. Each cell may include four fuel assemblies 10 having adjacent fuel channels 12. Other fuel assemblies 10 may be placed in the reactor core outside of cells 40 and not adjacent to control blades. The fuel assemblies 10 in FIG. 2 are shown in section to illustrate control blades 45, which are conventionally cruciform-shaped and movably-positioned between the adjacent surfaces of the fuel channels 12 in a cell 40 for purposes of controlling the reaction rate of the reactor core. Conventionally, there is one control blade 45 per cell 40. As a result, each fuel channel 12 has two sides adjacent to the control blade 45 and two sides with no adjacent control blade.
The control blade 45 is formed of materials that are capable of absorbing neutrons without undergoing fission itself, for example, boron, hafnium, silver, indium, cadmium, or other elements having a sufficiently high capture cross section for neutrons. Thus, when the control blade 45 is moved between the adjacent surfaces of the fuel channels 12, the control blade 45 absorbs neutrons which would otherwise contribute to the fission reaction in the core. On the other hand, when the control blade 45 is moved out of the way, more neutrons will be allowed to contribute to the fission reaction in the core. Conventionally, only a fraction of all control blades 45 within a core will be exercised to control the fission reaction within the core during an operating cycle. As such, only a corresponding fraction of fuel assemblies will be directly adjacent to an extended control blade, or “subject to control,” during an operating cycle.
After a period of time, a fuel channel 12 may become distorted as a result of differential irradiation growth, differential hydrogen absorption, and/or irradiation creep. Differential irradiation growth is caused by fluence gradients and results in fluence-gradient bow. Differential hydrogen absorption is a function of differential corrosion resulting from shadow corrosion on the channel sides adjacent to the control blades 45 and the percent of hydrogen liberated from the corrosion process that is absorbed into the fuel channel 12; this results in shadow corrosion-induced bow. Irradiation creep is caused by a pressure drop across the channel faces, which results in permanent distortion called creep bulge. As a result, the distortion (bow and bulge) of the fuel channel 12 may interfere with the movement of the control blade 45. Channel/control blade interference may cause uncertainty in control blade location, increased loads on reactor structural components, and decreased scram velocities. Conventionally, if channel/control blade interference has become severe, the control blade is declared inoperable and remains fully inserted.